Underfill detection system for a biosensor

ABSTRACT

A biosensor has an underfill detection system that determines whether a sample of a biological fluid is large enough for an analysis of one or more analytes. The underfill detection system applies an excitation signal to the sample, which generates an output signal in response to the excitation signal. The underfill detection system switches the amplitude of the excitation signal. The transition of the excitation signal to a different amplitude changes the output signal when the sample is not large enough for an accurate and/or precise analysis. The underfill detection system measures and compares the output signal with one or more underfill thresholds to determine whether an underfill condition exists.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Application No. 13/107,363,titled “Underfill Detection System For A Biosensor,” filed May 13, 2011,now allowed, which is a continuation of U.S. Application No. 12/251,709,titled “Underfill Detection System for a Biosensor,” filed Oct. 15,2008, now issued as U.S. Pat. No. 7,966,859, which is a continuation ofPCT/US2007/068034, titled “Underfill Detection System for a Biosensor,”filed May 2, 2007, which was published in English and claims the benefitof and priority from U.S. Provisional Application No. 60/797,128, titled“Underfill Detection System for a Biosensor,” filed May 3, 2006, each ofwhich is incorporated by reference in its entirety.

BACKGROUND

Biosensors usually provide an analysis of a biological fluid, such aswhole blood, urine, or saliva. Typically, a biosensor analyzes a sampleof the biological fluid to determine the concentration of one or moreanalytes, such as glucose, uric acid, lactate, cholesterol, orbilirubin, in the biological fluid. The analysis is useful in thediagnosis and treatment of physiological abnormalities. For example, adiabetic individual may use a biosensor to determine the glucose levelin blood for adjustments to diet and/or medication. When used, abiosensor may be underfilled if the sample size is not large enough. Anunderfilled biosensor may not provide an accurate analysis of thebiological fluid.

Biosensors may be implemented using bench-top, portable, and likedevices. The portable devices may be hand-held. Biosensors may bedesigned to analyze one or more analytes and may use different volumesof biological fluids. Some biosensors may analyze a single drop of wholeblood, such as from 0.25-15 microliters (μL) in volume. Examples ofportable measuring devices include the Ascensia Breeze® and Elite®meters of Bayer Corporation; the Precision® biosensors available fromAbbott in Abbott Park, Ill.; Accucheck® biosensors available from Rochein Indianapolis, Ind.; and OneTouch Ultra® biosensors available fromLifescan in Milpitas, Calif. Examples of bench-top measuring devicesinclude the BAS 100B Analyzer available from BAS Instruments in WestLafayette, Ind.; the CH Instruments' Electrochemical Workstationavailable from CH Instruments in Austin, Tex.; the CypressElectrochemical Workstation available from Cypress Systems in Lawrence,Kans.; and the EG&G Electrochemical Instrument available from PrincetonResearch Instruments in Princeton, N.J.

Biosensors usually measure an electrical signal to determine the analyteconcentration in a sample of the biological fluid. The analyte typicallyundergoes an oxidation/reduction or redox reaction when an excitationsignal is applied to the sample. An enzyme or similar species may beadded to the sample to enhance the redox reaction. The excitation signalusually is an electrical signal, such as a current or potential. Theredox reaction generates an output signal in response to the excitationsignal. The output signal usually is an electrical signal, such as acurrent or potential, which may be measured and correlated with theconcentration of the analyte in the biological fluid.

Many biosensors have a measuring device and a sensor strip. A sample ofthe biological fluid is introduced into a sample chamber in the sensorstrip. The sensor strip is placed in the measuring device for analysis.The measuring device usually has electrical contacts that connect withelectrical conductors in the sensor strip. The electrical conductorstypically connect to working, counter, and/or other electrodes thatextend into a sample chamber. The measuring device applies theexcitation signal through the electrical contacts to the electricalconductors in the sensor strip. The electrical conductors convey theexcitation signal through the electrodes into a sample deposited in thesample chamber. The redox reaction of the analyte generates an outputsignal in response to the excitation signal. The measuring devicedetermines the analyte concentration in response to the output signal.

The sensor strip may include reagents that react with the analyte in thesample of biological fluid. The reagents may include an ionizing agentfor facilitating the redox of the analyte, as well as mediators or othersubstances that assist in transferring electrons between the analyte andthe conductor. The ionizing agent may be an analyte specific enzyme,such as glucose oxidase or glucose dehydrogenase, which catalyze theoxidation of glucose in a whole blood sample. The reagents may include abinder that holds the enzyme and mediator together.

Biosensors may include an underfill detection system to prevent orscreen out analyses associated with sample sizes that are ofinsufficient volume. Because concentration values obtained from anunderfilled sensor strip may be inaccurate, the ability to prevent orscreen out these inaccurate analyses may increase the accuracy of theconcentration values obtained. Some underfill detection systems have oneor more indicator electrodes that detect the partial and/or completefilling of a sample chamber within a sensor strip. The indicatorelectrode(s) may be separate or part of the working, counter, or otherelectrodes used to determine the concentration of analyte in the sample.An electrical signal usually passes through the indicator electrode(s)when a sample is present in the sample chamber. The electrical signalmay be used to indicate whether a sample is present and whether thesample partially or completely fills the sample chamber.

Some biosensors have a third or indicator electrode in addition to thecounter and working electrodes used to apply an excitation signal to asample of the biological fluid. The third electrode may be positioned todetect whether the sample forms a liquid junction between theelectrodes. In operation, a potential is applied between the thirdelectrode and the counter electrode. When the sample connects theelectrodes, current flows between the third and counter electrodes. Thebiosensor detects the current to determine whether the sensor strip isfilled. A biosensor using an underfill detection system with a thirdelectrode is described in U.S. Pat. No. 5,582,697.

Other biosensors use a sub-element of the counter electrode to determinewhether the sensor strip is underfilled. The sub-element may be locatedupstream from the working electrode, where only the sub-element is inelectrical communication with the working electrode when the sensorstrip is underfilled. In operation, an insufficient flow of currentbetween the sub-element and the working electrode occurs when the sensorstrip is underfilled. The biosensor detects the insufficient flow ofcurrent and provides an error signal indicating the sensor strip isunderfilled. A biosensor using an underfill detection system with asub-element of the counter electrode is described in U.S. Pat. No.6,531,040.

While these underfill detection systems balance various advantages anddisadvantages, none are ideal. These systems usually include additionalcomponents, such as the indicator electrodes. The additional componentsmay increase the manufacturing cost of the sensor strip. The additionalcomponents also may introduce additional inaccuracy and imprecision dueto the variability of manufacturing processes.

In addition, these systems may require a larger sample chamber toaccommodate the indicator electrodes. The larger sample chamber mayincrease the sample size needed for an accurate and precise analysis ofthe analyte.

Moreover, these systems may be affected by uneven or slow filling of thesample chamber. The uneven or slow filling may cause these systems toindicate that the sensor strip is underfilled when the sample size islarge enough. The uneven or slow filling also may cause these systems toindicate the sensor strip is filled when the sample size is not largeenough.

These systems also may not detect that the sensor strip is underfilledearly enough to add more of the biological fluid. The delay may requirereplacing the sensor strip with a new sensor strip and a new sample ofthe biological fluid.

Accordingly, there is an ongoing need for improved biosensors,especially those that may provide increasingly accurate and/or precisedetection of underfilled sensor strips. The systems, devices, andmethods of the present invention overcome at least one of thedisadvantages associated with conventional biosensors.

SUMMARY

A biosensor with an underfill detection system determines whether asample of a biological fluid is large enough for an analysis of one ormore analytes. The underfill detection system measures a test outputsignal from the sample in response to test excitation signal. Theunderfill detection system switches the test excitation signal to one ormore different amplitudes. The transition to one or more differentamplitudes changes the test output signal from the sample in response toan underfill condition.

In a method for detecting an underfill condition in a biosensor, a testexcitation signal is applied to a sample of a biological fluid. The testexcitation signal is switched to one or more different amplitudes. Atest output signal from the sample is measured. The test output signalis compared with one or more underfill thresholds.

In another method for detecting an underfill condition in a biosensor, apolling excitation signal is applied to a sample of a biological fluid.A test excitation signal is applied to the sample when a polling outputsignal from the sample is equal to or greater than a polling threshold.The test excitation signal is switched to one or more differentamplitudes. A test output signal from the sample is measured. The testoutput signal is compared with one or more underfill thresholds. Anerror signal is generated.

A biosensor for determining an analyte concentration in a biologicalfluid may have a sensor strip and a measuring device. The sensor stripmay have a sample interface on a base. The sample interface is adjacentto a reservoir formed by the base. The measuring device may have aprocessor connected to a sensor interface. The sensor interface may haveelectrical communication with the sample interface. The processorapplies a test excitation signal to the sample interface. The processorswitches the test excitation signal to one or more different amplitudes.The processor measures a test output signal from the sample interface.The processor compares the test output signal to one or more underfillthresholds.

A method, for detecting an underfill condition in a biosensor, includesapplying a test excitation signal to a sample of a biological fluid,switching the test excitation signal to at least one differentamplitude, measuring a test output signal from the sample, and comparingthe test output signal with at least one underfill threshold. The testexcitation signal may be part of an assay excitation signal in anelectrochemical sensor system. The test excitation signal may have atest pulse width in the range of about 0.1 sec through about 3 sec and atest pulse interval in the range of about 0.2 sec through about 6 sec.The method may apply the test excitation signal during a test period ofless than about 180 sec. The test period may be in the range of about 1sec through about 100 sec. The method may include applying the testexcitation signal during a test period having test pulse intervals inthe range of about 2 through about 50.

The detection method may include switching the test excitation signal toat least one different amplitude essentially at a start of the testexcitation signal, to at least one different amplitude during a testpulse, and/or to at least one different amplitude during a transitionfrom one test pulse to another test pulse. The method may includeswitching the test excitation signal to a first different amplitudeduring a test pulse and switching the test excitation signal to a seconddifferent amplitude during a transition from one test pulse to anothertest pulse. The method may include decreasing the amplitude essentiallyat the start of the test excitation signal, decreasing the amplitude ofthe test excitation signal during the transition from one test pulse toanother test pulse, and/or decreasing the amplitude the test excitationsignal multiple times.

The detection method may include generating a decrease in the testoutput signal in response to an underfill condition and/or generating anerror signal in response to an underfill condition. The method mayrequest the addition of biological fluid to the sample in response tothe error signal and/or stop the analysis.

The detection method may include detecting when a sample of a biologicalfluid is available for analysis and may apply a polling excitationsignal to the sample. The test excitation signal may be switched to adifferent amplitude than the polling excitation signal. A polling outputsignal may be generated in response to the polling excitation signal andthe test excitation signal may be applied to the sample when the pollingoutput signal is equal to or greater than a polling threshold.

The at least one different amplitude of the method may be lower than anoriginal amplitude. The original and different amplitudes may beselected from an output signal plateau in an electrochemical sensorsystem. The output signal plateau may include excitation amplitudes thatgenerate output signals within ±5% of an average output signal.

A method for detecting an underfill condition in a biosensor includesapplying a polling excitation signal to a sample of a biological fluid,applying a test excitation signal to the sample when a polling outputsignal from the sample is equal to or greater than a polling threshold,switching the test excitation signal to at least one differentamplitude, measuring a test output signal from the sample, comparing thetest output signal with at least one underfill threshold, and generatingan error signal. The polling excitation signal may have a polling pulsewidth of less than about 300 ms and a polling pulse interval of lessthan about 1 sec. The polling excitation signal may have a polling pulsewidth in the range of about 0.5 ms through about 75 ms and may have apolling pulse interval in the range of about 5 ms through about 300 ms.The test excitation signal may have a test pulse width of less thanabout 5 sec and a test pulse interval of less than about 15 sec. Thetest excitation signal may have a test pulse width in the range of about0.1 sec through about 3 sec and have a test pulse interval in the rangeof about 0.2 sec through about 6 sec. The polling excitation signal mayhave at least one polling pulse with an amplitude of about 400 mV, andthe test excitation signal may have at least one test pulse with anamplitude of about 200 mV.

The at least one different amplitude may be lower than an originalamplitude and the original amplitude may be an amplitude of the pollingexcitation signal. The original and different amplitudes may be selectedfrom an output signal plateau in an electrochemical sensor system. Theoutput signal plateau may include excitation amplitudes that generateoutput signals within ±5% of an average output signal.

The method also may include applying the polling excitation signalduring a polling period of less than about 180 sec and applying the testexcitation signal during a test period of less than about 180 sec. Thismethod may include applying the polling excitation signal during apolling period in the range of about 0.1 sec through about 10 sec andapplying the test excitation signal during a test period in the range ofabout 1 sec through about 100 sec.

The method also may include switching the test excitation signal to atleast one different amplitude essentially at a start of the testexcitation signal, switching the test excitation signal to at least onedifferent amplitude during a test pulse, and/or switching the testexcitation signal to at least one different amplitude during atransition from one test pulse to another test pulse. The method alsomay include switching the test excitation signal to a first differentamplitude during a test pulse and switching the test excitation signalto a second different amplitude during a transition from one test pulseto another test pulse.

The method also may include decreasing the amplitude essentially at thestart of the test excitation signal, decreasing the amplitude of thetest excitation signal during the transition from one test pulse toanother test pulse, and/or decreasing the amplitude the test excitationsignal multiple times. The method also may include generating a decreasein the test output signal in response to an underfill condition,decreasing the test output signal in response to an underfill condition,and/or generating a negative test output signal in response to anunderfill condition.

The test output signal may indicate an underfill condition when the testoutput signal is equal to or less than a first underfill threshold, andwhere the test output signal indicates an underfill condition when achange in the test output signal is equal to or greater than a secondunderfill threshold. The method also may include requesting the additionof biological fluid to the sample in response to the error signal and/orstopping an analysis of an analyte in the sample in response to theerror signal. The test excitation signal may be part of an assayexcitation signal in an electrochemical sensor system.

A biosensor, for determining an analyte concentration in a biologicalfluid, including a sensor strip having a sample interface on a base,where the sample interface is adjacent to a reservoir formed by thebase, a measuring device having a processor connected to a sensorinterface, where the sensor interface has electrical communication withthe sample interface, and where the processor applies a test excitationsignal to the sample interface, the processor switches the testexcitation signal to at least one different amplitude, the processormeasures a test output signal from the sample interface, and theprocessor compares the test output signal to at least one underfillthreshold. The processor may apply a polling excitation signal to thesample. The processor may switch from the polling excitation signal tothe test excitation signal when the polling output signal is equal to orgreater than a polling threshold. The processor may apply the pollingexcitation signal during a polling period of less than 180 seconds andmay apply the test excitation signal during a test period of less than180 seconds.

The polling excitation signal may have a polling pulse width in therange of about 0.5 ms through about 75 ms and the polling excitationsignal may have a polling pulse interval in the range of about 5 msthrough about 300 ms. The test excitation signal may have a test pulsewidth less than about 5 sec and a test pulse interval less than about 15sec. The at least one different amplitude may be lower than an originalamplitude. The original amplitude may be an amplitude of a pollingexcitation signal. The original and different amplitudes may be selectedfrom an output signal plateau in an electrochemical sensor system andthe output signal plateau may include excitation amplitudes thatgenerate output signals within ±5% of an average output signal.

The processor of the biosensor may switch the test excitation signal toat least one different amplitude essentially at a start of the testexcitation signal. The processor may switch the test excitation signalto at least one different amplitude during a test pulse and/or mayswitch the test excitation signal to at least one different amplitudeduring a transition from one test pulse to another test pulse. Theprocessor may switch the test excitation signal to a first differentamplitude during a test pulse and may switch the test excitation signalto a second different amplitude during a transition from one test pulseto another test pulse. The processor may reduce the amplitude of atleast one test pulse in the test excitation signal below the amplitudeof a polling pulse in the polling excitation signal. The test excitationsignal may be part of an assay excitation signal in an electrochemicalsensor system.

The biosensor may include a display connected to the processor, wherethe processor shows an error signal on the display in response to anunderfill condition. The error signal may request the user to addbiological fluid to the sample in response to the error signal and/orthe processor may stop the analysis of the analyte in the sample inresponse to the error signal. The sample interface may have a counterelectrode and a working electrode, the counter electrode may have asub-element.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. Moreover, in the figures, likereferenced numerals designate corresponding parts throughout thedifferent views.

FIG. 1 represents a method for detecting an underfill condition in abiosensor.

FIG. 2 is a graph illustrating a semi-integral of a cyclic voltammogramfor a ferri/ferrocyanide redox couple.

FIG. 3 is a graph illustrating an amplitude reduction at the beginningof the test excitation signal.

FIG. 4 is a graph illustrating a first amplitude reduction at the startof the first test pulse and a second amplitude reduction between thefirst and second pulses of the test excitation signal.

FIG. 5 is a graph illustrating an amplitude reduction between the firstand second pulses of the test excitation signal.

FIG. 6 is a graph illustrating another amplitude reduction between thefirst and second pulses of the test excitation signal.

FIG. 7 is a graph illustrating a first amplitude reduction within thefirst test pulse and a second amplitude reduction between the first andsecond pulses of the test excitation signal.

FIG. 8 is a graph illustrating the test output signal in relation topolling and test excitation signals.

FIG. 9 is a graph illustrating the test output signals of underfilledand filled conditions when the amplitude is reduced at the beginning ofthe test excitation signal.

FIG. 10 is a graph illustrating the test output signals of underfilledand filled conditions when a first amplitude reduction occurs at thebeginning of the first test pulse and a second amplitude reductionoccurs between the first and second test pulses of the test excitationsignal.

FIG. 11 is a graph illustrating the test output signals of underfilledand filled conditions when the amplitude of the test pulse is reducedbetween the first and second pulses.

FIG. 12 is a graph illustrating other test output signals of underfilledand filled conditions when the amplitude of the test pulse is reducedbetween the first and second pulses.

FIG. 13 is a graph illustrating the percent bias of analyte analyses inrelation to the volume of a sample.

FIG. 14 is a graph illustrating the percent population of differenttypes of test output signals in relation to the volume of a sample forthe analyte analyses of FIG. 11.

FIG. 15 depicts a schematic representation of a biosensor with anunderfill detection system.

DETAILED DESCRIPTION

The present invention provides an underfill detection system for abiosensor. The underfill detection system improves the accuracy andprecision of the biosensor in determining whether a sample of abiological fluid is large enough for an analysis of one or moreanalytes. The underfill detection system applies a test excitationsignal to a sample deposited in the biosensor. The test excitationsignal is switched to one or more different amplitudes. The samplegenerates a test output signal in response to the test excitationsignal. The transition of the test excitation signal to a differentamplitude changes the test output signal when the sample is not largeenough for an accurate and/or precise analysis. The underfill detectionsystem measures and compares the test output signal with one or moreunderfill thresholds to determine whether an underfill condition exists.The biosensor may be utilized to determine one or more analyteconcentrations, such as glucose, uric acid, lactate, cholesterol,bilirubin, or the like, in a biological fluid, such as whole blood,urine, saliva, or the like.

FIG. 1 represents a method for detecting an underfill condition in abiosensor. In 102, the biosensor detects when a sample of a biologicalfluid is available for analysis. In 104, the biosensor applies a testexcitation signal to the sample. In 106, the biosensor switches the testexcitation signal to at least one different amplitude. In 108, thebiosensor measures the test output signal generated by the sample inresponse to the test excitation signal. In 110, the biosensor comparesthe test output signal with one or more underfill thresholds. In 112,the biosensor generates an error signal or other indication in responseto an underfill condition when the test output signal indicates thesample size is not large enough.

In 102 of FIG. 1, the biosensor detects when a sample of a biologicalfluid is available for analysis. The biosensor may sense when a sensorstrip is placed in a measuring device. The biosensor may sense(mechanically, electrically, or the like) when electrical contacts inthe measuring device connect with electrical conductors in the sensorstrip. The biosensor may apply a polling excitation signal or othersensing signal to the working, counter, and/or other electrodes todetect when a sample connects with the electrodes. The biosensor may useother methods and devices to detect when a sample is available foranalysis.

The polling excitation signal is an electrical signal, such as currentor potential, that pulses or turns on and off at a set frequency orinterval. The sample generates a polling output signal in response tothe polling excitation signal. The polling output signal is anelectrical signal, such as current or potential. The biosensor may showthe polling output signal on a display and/or may store the test outputsignal in a memory device.

The polling excitation signal is a sequence of polling pulses separatedby polling relaxations. During a polling pulse, the electrical signal ison. During a polling relaxation, the electrical signal is off. On mayinclude time periods when an electrical signal is present. Off mayinclude time periods when an electrical signal is not present. Off maynot include time periods when an electrical signal is present but hasessentially no amplitude. The electrical signal may switch between onand off by closing and opening an electrical circuit, respectively. Theelectrical circuit may be opened and closed mechanically, electrically,or the like.

A polling excitation signal may have one or more polling pulseintervals. A polling pulse interval is the sum of a polling pulse and apolling relaxation. Each polling pulse has an amplitude and a pollingpulse width. The amplitude indicates the intensity of the potential, thecurrent, or the like of the electrical signal. The amplitude may vary orbe a constant during the polling pulse. The polling pulse width is thetime duration of a polling pulse. The polling pulse widths in a pollingexcitation signal may vary or be essentially the same. Each pollingrelaxation has a polling relaxation width, which is the time duration ofa polling relaxation. The polling relaxation widths in a pollingexcitation signal may vary or be essentially the same.

The polling excitation signal may have a polling pulse width of lessthan about 300 milliseconds (ms) and a polling pulse interval of lessthan about 1 sec. The polling excitation signal may have a polling pulsewidth of less than about 100 ms and a polling pulse interval of lessthan about 500 ms. The polling excitation signal may have a pollingpulse width in the range of about 0.5 ms through about 75 ms and apolling pulse interval in the range of about 5 ms through about 300 ms.The polling excitation signal may have a polling pulse width in therange of about 1 ms through about 50 ms and a polling pulse interval inthe range of about 10 ms through about 250 ms. The polling excitationsignal may have a polling pulse width of about 5 ms and a polling pulseinterval of about 125 ms. The polling excitation signal may have otherpulse widths and pulse intervals.

The biosensor may apply the polling excitation signal to the sampleduring a polling period. The polling period may be less than about 15minutes, 5 minutes, 2 minutes, or 1 minute. The polling period may belonger depending upon how a user uses the biosensor. The polling periodmay be in the range of about 0.5 second (sec) through about 15 minutes.The polling period may be in the range of about 5 sec through about 5minutes. The polling period may be in the range of about 10 sec throughabout 2 minutes. The polling period may be in the range of about 20 secthrough about 60 sec. The polling period may be in the range of about 30through about 40 sec. The polling period may have less than about 200,100, 50, or 25 pulse intervals. The polling period may have from about 2through about 150 pulse intervals. The polling period may have fromabout 5 through about 50 pulse intervals. The polling period may havefrom about 5 through about 15 pulse intervals. The polling period mayhave about 10 pulse intervals. Other polling periods may be used.

In 104 of FIG. 1, the biosensor applies a test excitation signal to thesample. The biosensor applies the test excitation signal when thepolling output signal is equal to or greater than a polling threshold.The polling threshold may be greater than about 5 percent (%) of theexpected test excitation signal at the beginning of the first pulse. Thepolling threshold may be greater than about 15% of the expected testexcitation signal at the beginning of the first pulse. The pollingthreshold may be in the range of about 5 percent (%) through about 50%of the expected test excitation signal at the beginning of the firstpulse. Other polling thresholds may be used. The biosensor may indicatethe polling output signal is equal to or greater than the pollingthreshold on a display.

The test excitation signal is an electrical signal, such as current orpotential, that pulses or turns on and off at a set frequency orinterval. The sample generates a test output signal in response to thetest excitation signal. The test output signal is an electrical signal,such as current or potential.

The test excitation signal is a sequence of test pulses separated bytest relaxations. During a test pulse, the electrical signal is on.During a test relaxation, the electrical signal is off. On includes timeperiods when an electrical signal is present. Off includes time periodswhen an electrical signal is not present and does not include timeperiods when an electrical signal is present but has essentially noamplitude. The electrical signal switches between on and off by closingand opening an electrical circuit, respectively. The electrical circuitmay be opened and closed mechanically, electrically, or the like.

A test excitation signal may have one or more test pulse intervals. Atest pulse interval is the sum of a test pulse and a test relaxation.Each test pulse has an amplitude and a test pulse width. The amplitudeindicates the intensity of the potential, the current, or the like ofthe electrical signal. The amplitude may vary or be a constant duringthe test pulse. The test pulse width is the time duration of a testpulse. The test pulse widths in a test excitation signal may vary or beessentially the same. Each test relaxation has a test relaxation width,which is the time duration of a test relaxation. The test relaxationwidths in a test excitation signal may vary or be essentially the same.

The test excitation signal may have a test pulse width of less thanabout 5 sec and a test pulse interval of less than about 15 sec. Thetest excitation signal may have a test pulse width of less than about 3,2, 1.5, or 1 sec and a test pulse interval of less than about 13, 7, 4,3, 2.5, or 1.5 sec. The test excitation signal may have a test pulsewidth in the range of about 0.1 sec through about 3 sec and a test pulseinterval in the range of about 0.2 sec through about 6 sec. The testexcitation signal may have a test pulse width in the range of about 0.1sec through about 2 sec and a test pulse interval in the range of about0.2 sec through about 4 sec. The test excitation signal may have a testpulse width in the range of about 0.1 sec through about 1.5 sec and atest pulse interval in the range of about 0.2 sec through about 3.5 sec.The test excitation signal may have a test pulse width in the range ofabout 0.4 sec through about 1.2 sec and a test pulse interval in therange of about 0.6 sec through about 3.7 sec. The test excitation signalmay have a test pulse width in the range of about 0.5 sec through about1.5 sec and a test pulse interval in the range of about 0.75 sec throughabout 2.0 sec. The test excitation signal may have a test pulse width ofabout 1 sec and a test pulse interval of about 1.5 sec. The testexcitation signal may have other pulse widths and pulse intervals.

The biosensor applies the test excitation signal to the sample during atest period. The test period may have the same or a different durationthan the polling period. The test excitation signal may be part of anassay excitation signal used in an electrochemical sensor system. Thetest excitation signal and the assay excitation signal may beessentially the same signal. The test period of the test excitationsignal may have the same or different duration as the assay excitationsignal.

The test period of the test excitation signal may be less than about180, 120, 90, 60, 30, 15, 10, or 5 sec. The test period may be in therange of about 1 sec through about 100 sec. The test period may be inthe range of about 1 sec through about 25 sec. The test period may be inthe range of about 1 sec through about 10 sec. The test period may be inthe range of about 2 sec through about 3 sec. The test period may beabout 2.5 sec. The test period may have less than about 50, 25, 20, 15,10, 8, 6, or 4 test pulse intervals. The test period may have test pulseintervals in the range of about 2 through about 50. The test period mayhave test pulse intervals in the range of about 2 through about 25. Thetest period may have test pulse intervals in the range of about 2through about 15. The test period may have about 10 test pulseintervals. Other test periods may be used.

In 106 of FIG. 1, the biosensor switches the test excitation signal toat least one different amplitude. When switching to a differentamplitude, the biosensor may apply a test excitation signal with adifferent amplitude than the amplitude of the polling excitation signal.When switching to a different amplitude, the biosensor may apply a testexcitation signal having one or more test pulses with differentamplitudes. When switching to a different amplitude, the biosensor mayapply a test excitation signal having one or more test pulses where theamplitude varies or shifts between different amplitudes. The biosensormay switch the amplitude of the test excitation signal essentially whenthe biosensor switches from the polling excitation signal to the testexcitation signal. The biosensor may switch the amplitude of the testexcitation signal essentially at the start of the test excitationsignal. The biosensor may switch the test excitation signal to adifferent amplitude during a test pulse, during the transition from onetest pulse to another test pulse, or the like. During a test pulseincludes the start of the test pulse, the end of the test pulse, and anyportion in between the start and end of the test pulse. During a testpulse includes any position or time from the start of the test pulse tothe end of the test pulse. During a transition from one test pulse toanother test pulse includes any position or time from the end of onetest pulse to the start of another test pulse. During a transition fromone test pulse to another test pulse includes any position or time thatis part of or include in a test relaxation. The biosensor may switch theamplitude of the test excitation signal multiple times. The biosensormay switch the test output signal to a first different amplitude andlater switch to a second different amplitude. Other switches in theamplitude of the test excitation signal may occur.

A different amplitude may be any amplitude that is essentially not thesame as an original amplitude. The different amplitude may be higher orlower than the original amplitude. The different amplitude is theamplitude of the test excitation signal after a switch has occurred. Anoriginal amplitude is the amplitude prior to the switch. The originalamplitude may be the amplitude of the polling excitation signal, thefirst or another test pulse in the test excitation signal, or the like.Other original and different amplitudes may be used.

A higher amplitude may be up to about 400% greater than the originalamplitude. A higher amplitude may be in the range of about 2% throughabout 200% greater than the original amplitude. A higher amplitude maybe in the range of about 5% through about 100% greater than the originalamplitude. A higher amplitude may be in the range of about 25% throughabout 75% greater than the original amplitude. A higher amplitude may beabout 50% greater than the original amplitude. Other higher amplitudesmay be used.

A lower amplitude may be in the range of about 2% through about 98% lessthan the original amplitude. A lower amplitude may be in the range ofabout 5% through about 95% less than the original amplitude. A loweramplitude may be in the range of about 10% through about 90% less thanthe original amplitude. A lower amplitude may be in the range of about20% through about 80% less than the original amplitude. A loweramplitude may be in the range of about 25% through about 65% less thanthe original amplitude. A lower amplitude may be about 50% less than theoriginal amplitude. Other lower amplitudes may be used.

Each switch to a different amplitude may generate a change in the testoutput signal in response to an underfill condition. The change in thetest output signal may include test output signals that are or becomestronger or weaker than the test output signal when there is nounderfill condition. The change in the test output signal may occuressentially at the same time and/or after the switch to a differentamplitude occurs. The change in the test output signal may be measurableand may last for more than about 1 sec. When the amplitude of the testexcitation signal is changed multiple times, each transition from or toa different amplitude may generate a further change in the test outputsignal.

The change in the test output signal may be a shift to the stronger orweaker test output signal. The shift may be essentially instantaneous,gradual, a combination thereof, or the like. A stronger test outputsignal has a greater or higher intensity than a weaker test outputsignal. For example, a test output signal of 2000 nanoAmperes (nA) isstronger or greater than a test output signal of 1200 nA. For example, atest output signal of −1100 nA is weaker or less than a test outputsignal of 1000 nA. Other test output signals may be used.

A switch to a lower amplitude may generate a decrease in the test outputsignal in response to an underfill condition. A decrease in the testoutput signal may occur essentially at the start of the test outputsignal such as when the test excitation signal starts or when thepolling excitation signal switches to the test excitation signal. Adecrease in the test output signal may occur when the test output signalbecomes weaker or less after a switch of the test excitation signal to adifferent amplitude. The switch to a lower amplitude may generate anegative test output signal or a test output signal that becomesnegative.

A switch to a higher amplitude may generate an increase in the testoutput signal in response to an underfill condition. An increase in thetest output signal may occur essentially at the start of the test outputsignal such as when the test excitation signal starts or when thepolling excitation signal switches to the test excitation signal. Anincrease in the test output signal may occur when the test output signalbecomes stronger or greater after a switch of the test excitation signalto a different amplitude.

The original and different amplitudes may be selected to provide a moremeasurable or cleaner change in the test output signal when an underfillcondition exists. The original and different amplitudes may selected toprovide a change in the test output signal that is more independent ofother conditions during the analysis of the sample. The original anddifferent amplitudes may be selected so there is little or no change inthe redox reaction of the analyte in the sample when amplitudetransitions occur. In addition, the difference in the original anddifferent amplitudes may be selected to increase or decrease thereduction in the test output signal when an underfill condition exists.

The original and different amplitudes may be selected from excitationamplitudes within an output signal plateau of the mediator in anelectrochemical sensor system. A switch from one excitation amplitude toanother excitation amplitude in the output signal plateau may generatelittle or no change in the redox reaction of the analyte in the sample.The output signal plateau may include excitation amplitudes where theelectrochemical sensor system generates essentially the same or constantoutput signals. The output signal plateau may include excitationamplitudes where the electrochemical sensor system generates outputsignals within 1% of an average output signal or a selected outputsignal for the output signal plateau. The output signal plateau mayinclude excitation amplitudes where the electrochemical sensor systemgenerates output signals within ±5% of an average output signal or aselected output signal for the output signal plateau. The output signalplateau may include excitation amplitudes where the electrochemicalsensor system generates output signals within ±10% of an average outputsignal or a selected output signal for the output signal plateau. Otheroutput signal plateaus may be used.

FIG. 2 is a graph illustrating a semi-integral of a cyclic voltammogramfor a ferri/ferrocyanide redox couple compared against the sameferri/ferrocyanide redox couple at the counter electrode. Thesemi-integral represents the current as a function of the appliedpotential in an electrochemical sensor system using a voltammetry orgated voltammetry electrochemical sensor system. The ferri/ferrocyanideredox couple is a mediator that assists with the oxidation and reductionof the analyte in the sample. Other redox couples may be used.

The semi-integral defines a current plateau in a range from about 0.18volts (V) through about 0.6 V, where the current is essentially constantat about 27 micro-Coulombs per the square root of seconds(μCoul/sec^(1/2)). Within the current plateau, there is little or nochange in the faradaic reaction—the transfer of electrons between theanalyte and mediator and the electrodes in the biosensor. Only acharging current is generated due to the change in potential. Theoriginal and different amplitudes may be selected from potentials withinthe current plateau. An amplitude or potential of about 0.4 V (A in FIG.2) for the original amplitude may be selected. The original amplitudemay be the amplitude of polling pulse in a polling excitation signal ora test pulse in a test excitation signal. An amplitude or potential ofabout 0.2 V (B in FIG. 2) for the different amplitude may be selected.The different amplitude may be the amplitude of a test pulse or aportion of a test pulse in a test excitation signal. Other original anddifferent amplitudes may be selected from the current plateau.

In 104 and 106 of FIG. 1, the polling and test signals may be part of oran addition to an electrochemical or optical sensor system used todetermine one or more analyte concentrations in a sample of biologicalfluid. In electrochemical and optical sensor systems, anoxidation/reduction or redox reaction of an analyte in the samplegenerates an assay output signal. An enzyme or similar species may beadded to the sample to enhance the redox reaction. The assay outputsignal is measured and correlated to the concentration of the analyte inthe sample.

Optical sensor systems generally measure the amount of light absorbed orgenerated by the reaction of a chemical indicator with the analyte redoxreaction. An enzyme may be included with the chemical indicator toenhance the reaction kinetics. The assay output signal or light from anoptical system may be converted into an electrical signal such ascurrent or potential.

In light-absorption optical systems, the chemical indicator produces areaction product that absorbs light. An incident excitation beam from alight source is directed toward the sample. The incident beam may bereflected back from or transmitted through the sample to a detector. Thedetector collects and measures the attenuated incident beam (assayoutput signal). The amount of light attenuated by the reaction productis an indication of the analyte concentration in the sample.

In light-generated optical systems, the chemical detector fluoresces oremits light in response to the analyte redox reaction. A detectorcollects and measures the generated light (assay output signal). Theamount of light produced by the chemical indicator is an indication ofthe analyte concentration in the sample.

Electrochemical sensor systems apply an assay excitation signal to thesample of the biological fluid. The assay excitation signal may be apotential or current and may be constant, variable, or a combinationthereof such as when an AC signal is applied with a DC signal offset.The assay excitation signal may be applied as a single pulse or inmultiple pulses, sequences, or cycles. The analyte undergoes a redoxreaction when the assay excitation signal is applied to the sample. Anenzyme or similar species may be used to enhance the redox reaction ofthe analyte. A mediator may be used to maintain the oxidation state ofthe enzyme. The redox reaction generates an assay output signal that maybe measured constantly or periodically during transient and/orsteady-state output. Various electrochemical processes may be used suchas amperometry, coulometry, voltammetry, gated amperometry, gatedvoltammetry, and the like.

In amperometry, a potential or voltage is applied to a sample of thebiological fluid. The redox reaction of the analyte generates a currentin response to the potential. The current is measured at a fixed time ata constant potential to quantify the analyte in the sample. Amperometrygenerally measures the rate at which the analyte is oxidized or reducedto determine the analyte concentration in the sample. Biosensor systemsusing amperometry are described in U.S. Pat. Nos. 5,620,579; 5,653,863;6,153,069; and 6,413,411.

In coulometry, a potential is applied to a sample of the biologicalfluid to exhaustively oxidize or reduce the analyte within the sample.The potential generates a current that is integrated over the time ofoxidation/reduction to produce an electrical charge representing theanalyte concentration. Coulometry generally captures the total amount ofanalyte within the sample. A biosensor system using coulometry for wholeblood glucose measurement is described in U.S. Pat. No. 6,120,676.

In voltammetry, a varying potential is applied to a sample of biologicalfluid. The redox reaction of the analyte generates current in responseto the applied potential. The current is measured as a function ofapplied potential to quantify the analyte in the sample. Voltammetrygenerally measures the rate at which the analyte is oxidized or reducedto determine the analyte concentration in the sample. Additionalinformation about voltammetry may be found in “Electrochemical Methods:Fundamentals and Applications” by A. J. Bard and L. R. Faulkner, 1980.

In gated amperometry and gated voltammetry, pulsed excitations may beused as described in US Provisional Patent Application Nos. 60/700,787,filed Jul. 20, 2005, and 60/722,584, filed Sep. 30, 2005, respectively,which are incorporated by reference.

The test excitation and output signals may be incorporated with thepulsed excitation and output signals of an electrochemical sensorsystem. The test excitation signal may be part of the assay excitationsignal applied to a sample in gated amperometry or gated voltammetrysystems. The test excitation signal may be the portion of the assayexcitation signal that is applied to the sample during the test period.The test output signal may be the portion of the assay output signalgenerated by a sample during the test period. The test excitation andoutput signals may be incorporated with other electrochemical sensorsystems.

FIGS. 3-7 are graphs illustrating the polling and test excitationsignals for an underfill detection system. While a polling excitationsignal is shown, the underfill detection system may operate without apolling excitation signal. In FIGS. 3-5, there is little or no pollingrelaxation width between the last polling pulse of the pollingexcitation signal and the first test pulse of the test excitationsignal. In FIGS. 6-7, the polling relaxation width between the lastpolling pulse and the first test pulse may be the same or different thananother polling relaxation width in the polling excitation signal.

In FIGS. 3-7, the polling excitation signal has an amplitude of about400 mV. The test excitation signal has an amplitude that is reduced toabout 200 mV. The polling excitation signal has a polling pulse width ofabout 5 ms and a polling pulse interval of about 250 ms. The testexcitation signal has a test pulse width of about 1 sec and a test pulseinterval of about 1.5 sec. The test excitation signal may be a portionof the assay excitation signal for an electrochemical sensor system,such as gated amperometry, gated voltammetry, or the like. Other pollingand test excitation signals may be used.

FIG. 3 is a graph illustrating an amplitude reduction at the beginningof the test excitation signal. There is little or no polling relaxationwidth between the last polling pulse of the polling excitation signaland the first test pulse of the test excitation signal. The transitionfrom about 400 mV to about 200 mV occurs at about 0 sec, when thebiosensor switches from the polling excitation signal to the testexcitation signal.

FIG. 4 is a graph illustrating a first amplitude reduction at the startof the first test pulse and a second amplitude reduction between thefirst and second pulses of the test excitation signal. There is littleor no polling relaxation width between the last polling pulse of thepolling excitation signal and the first test pulse of the testexcitation signal. A first transition from about 400 mV to about 300 mVoccurs at about 0 sec, when the biosensor switches from the pollingexcitation signal to the test excitation signal. A second transitionfrom about 300 mV to about 200 mV occurs at about 1-1.5 sec, between thefirst and second pulses.

FIG. 5 is a graph illustrating an amplitude reduction of the test pulsebetween the first and second pulses of the test excitation signal. Thereis little or no polling relaxation width between the last polling pulseof the polling excitation signal and the first test pulse of the testexcitation signal. The transition from about 400 mV to about 200 mVoccurs at about 1-1.5 sec, between the first and second pulses.

FIG. 6 is a graph illustrating another amplitude reduction of the testpulse between the first and second pulses of the test excitation signal.The polling relaxation width between the last polling pulse and thefirst test pulse may be the same or different than another pollingrelaxation width in the polling excitation signal. The transition fromabout 400 mV to about 200 mV occurs at about 1-1.5 sec, between thefirst and second pulses.

FIG. 7 is a graph illustrating a first amplitude reduction within thefirst test pulse and a second amplitude reduction between the first andsecond pulses of the test excitation signal. The polling relaxationwidth between the last polling pulse and the first test pulse may be thesame or different than another polling relaxation width in the pollingexcitation signal. The first amplitude reduction occurs at about 0.5sec, when the biosensor switches the amplitude from about 400 mV toabout 300 mV in the first pulse. The second amplitude reduction occursat about 1-1.5 sec, when the biosensor switches the amplitude from about300 mV to about 200 mV between the first and second pulses.

In 108 of FIG. 1, the biosensor measures the test output signalgenerated by the sample. The sample generates the test output signal inresponse to the test excitation signal. The biosensor may show the testoutput signal on a display and/or may store test output signal in amemory device.

FIG. 8 is a graph illustrating the test output signal in relation topolling and test excitation signals. The sample of biological fluidessentially fills the sample chamber; in other words, no underfillcondition exists. When the sample chamber is essentially filled with thesample, the enzymatic and electrochemical reactions occur and the testoutput signal or current is generated in response to the test excitationsignal or potential as expected. Other polling and test excitationsignals may be used. Other test output signals may result includingthose that may decline initially and those that may decline in allpulses.

The polling excitation signal has an amplitude of about 400 mV with apolling pulse width of about 50 ms and a polling pulse interval of about250 ms. The test excitation signal has an initial amplitude of 400 mVthat is reduced to a final amplitude of about 200 mV. The testexcitation signal has a test pulse width of about 1 sec and a test pulseinterval of about 1.5 sec. The initial amplitude of the test excitationsignal is reduced to the final amplitude between the first and secondpulses. The transition from about 400 mV to about 200 mV occurs at about1-1.5 sec. The test excitation signal may be a portion of the assayexcitation signal for an electrochemical sensor system, such as gatedamperometry, gated voltammetry, and the like.

The sample generates current or the test output signal in response tothe applied potential or test excitation signal. The applied potentialof the first test pulse is about 400 mV, which is essentially the sameas the applied potential of the polling pulses. The current of the firsttest pulse increases from the beginning to the end of the pulse. Thetransition from a higher to lower potential occurs between the first andsecond test pulses. The applied potential of the second and followingtest pulses is about 200 mV. The current of the second and followingtest pulses is higher at the beginning of the test pulse than current atthe end of the previous test pulse. The current of the second andfollowing test pulses decreases from the beginning to the end of thepulse.

In FIG. 8, the test output signal and polling and test excitationsignals may be for a biosensor having a working electrode, a counterelectrode, and trigger electrode (which may be a sub-unit or sub-elementof the counter electrode). The biosensor may measure the concentrationof glucose in whole blood. Other biosensors may be used including thosewith additional electrodes and different configurations. Other analyteconcentrations may be measured including those in other biologicalfluids.

In use, a sensor strip is inserted into the sensor port of the biosensorand the power is turned-on. The biosensor applies the polling excitationsignal or polling potential to the working and counter electrodes of thesensor strip with the pulses having a pulse width of about 5-10 ms and apulse interval of about 125 ms. The biosensor waits for application ofthe sample (whole blood) to the sensor strip. The biosensor measures thepolling output signal. The biosensor may have a potentiostat thatprovides the polling output signal to the input of an analog comparator.

When there is only enough of the sample (whole blood) to cover thetrigger electrode and the working electrode, there may be a short burstof current under a polling excitation signal of about 400 mV. When theoutput signal is equal to or greater than a polling threshold value, thebiosensor applies the test excitation signal or potential to the workingand counter electrodes. The polling threshold valve may be about 250 nA.The test excitation signal may be part of the assay excitation signal inan electrochemical sensor system. The test and assay excitation signalsmay be essentially the same signal. The comparator may compare thepolling output signal to the polling threshold value. When the pollingoutput signal exceeds the polling threshold value, the output signal ofthe comparator may trigger the launch of the test excitation signal.

During the test excitation signal, the biosensor may apply a first testpulse having a potential of about 400 mV for about 1 sec to the workingand counter electrodes. The first test pulse is followed by a 0.5 sectest relaxation, which may be an essentially open circuit or the like.The test output signal or current within the first pulse is measured andstored in a memory device. The biosensor may apply a second pulse to theworking and counter electrodes at about 200 mV for about 1 sec. Thispotential switch from about 400 mV to about 200 mV may trigger anegative current if there is an insufficient sample in the sensor strip,especially when the sample covers only the working and triggerelectrodes. The test output signal or current within the second pulse ismeasured and stored in a memory device. The biosensor continues applyingtest pulses from the test excitation signal to the working and counterelectrodes until the end of the test period or for as long as desired bythe biosensor. The test period may be about 1 through about 10 sec. Thetest output signal or test current within each test pulse may bemeasured and stored.

The test output signals or test currents may be compared with one ormore filters to detect whether an underfill condition exists. Thefilters may be underfill thresholds where the test output signalsindicate there is not enough of the sample in the sensor strip. For afirst filter, any one of the test currents within a test pulse may becompared with a first underfill threshold value to detect whether anunderfill condition exists. For example, the current i_(2,8) at the endof the second test pulse may be compared with a first underfillthreshold of about 150 nA. For a second filter, the difference betweenthe two test currents may be compared to a second threshold value todetect whether an underfill condition exists. For example, thedifference between the last current in the first pulse i_(1,8) and thefirst current in the second pulse i_(2,1) may be compared to a secondthreshold value of about 700 nA. The filters may be used separately orin combination such as when the second filter detects an underfilledcondition that the first filter did not detect. When one of thefiltering conditions is met, the biosensor may provide an error signalor other indication to the user. The biosensor may stop applying thetest excitation signal and prompt the user to add more blood to thesensor strip. The user may be able to recover from the underfillcondition and avoid wasting a sensor strip.

FIGS. 9-12 are graphs illustrating the test output signals ofunderfilled and filled conditions. The underfilled conditions are forsamples of about 1.2 micro-Liters (μL). The filled conditions are forsamples of about 2.0 μL. The current profiles of the filled conditionsare similar to the current profile illustrated in FIG. 6. Theunderfilled conditions generated test output signals or current profileswith a negative current that dropped below about −1100 nA within about2.5 sec.

The test output signals or current profiles of the underfilled andfilled conditions are responsive to polling and excitation signals orapplied potentials. The polling excitation signal has an amplitude orpotential of about 400 mV with a polling pulse width of about 5 ms and apolling pulse interval of about 62.5 ms. The test excitation signal hasan amplitude that reduces to about 200 mV. The test excitation signalhas a test pulse width of about 1 sec and a test pulse interval of about1.5 sec. The polling excitation signal switches to the test excitationsignal when the polling output signal is equal to or greater than apolling threshold. The polling threshold may be about 250 nA. Otherpolling thresholds may be used. The test excitation signal may be aportion of an assay excitation signal for an electrochemical sensorsystem, such as gated amperometry, gated voltammetry, or the like.

FIG. 9 is a graph illustrating the test output signals of underfilledand filled conditions when the amplitude is reduced at the beginning ofthe test excitation signal. The samples are whole blood having a glucoseconcentration of about 50 milligrams per deciliter (mg/dL) and about 40%hematocrit. The amplitude reduction occurs at about 0 sec, when theamplitude switches from about 400 mV to about 200 mV at the beginning ofthe first test pulse. The underfilled condition generated a test outputsignal with a negative current during the first pulse of the testexcitation signal.

FIG. 10 is a graph illustrating the test output signals of underfilledand filled conditions when a first amplitude reduction occurs at thebeginning of the first test pulse and a second amplitude reductionoccurs between the first and second test pulses of the test excitationsignal. The samples are whole blood having a glucose concentration ofabout 50 mg/dL and about 40% hematocrit. The first amplitude reductionoccurs at about 0 sec, when the amplitude switches from about 400 mV toabout 300 mV at the beginning of the first test pulse. The secondamplitude reduction occurs at about 1-1.5 sec, when the amplitudeswitches from about 300 mV to about 200 mV between the first and secondpulses. The underfilled condition generated a test output signal with acurrent close to zero during the first pulse of the test excitationsignal; when the test pulse was reduced from about 400 mV to about 300mV. The underfilled condition generated a test output signal with anegative current during the second pulse of the test excitation signal;after the test pulse was reduced from about 300 mV to about 200 mV.

FIG. 11 is a graph illustrating the test output signals of underfilledand filled conditions when the amplitude is reduced between the firstand second test pulses of the test excitation signal. The samples arewhole blood having a glucose concentration of about 50 mg/dL and about40% hematocrit. The amplitude reduction occurs at about 1-1.5 sec; whenthe amplitude switches from about 400 mV to about 200 mV between thefirst and second test pulses. The underfilled condition generated a testoutput signal with a positive current during the first pulse of the testexcitation signal; when the applied potential of the test pulse remainedessentially the same as the applied potential of the polling pulse. Theunderfilled condition generated a test output signal with a negativecurrent during the second pulse of the test excitation signal; after thetest pulse was reduced from about 400 mV to about 200 mV.

FIG. 12 is a graph illustrating the test output signals of underfilledand filled conditions when the amplitude of the test pulse is reducedbetween the first and second pulses. The samples are whole blood havinga glucose concentration of about 400 mg/dl and about 40% hematocrit. Theamplitude reduction occurs at about 1-1.5 sec, when the amplitudeswitches from about 400 mV to about 200 mV. During the first pulse ofthe test excitation signal, the applied potential of the test pulseremained essentially the same as the applied potential of the pollingpulse. The underfilled condition generated a test output signal with apositive current during the first pulse. The underfilled conditiongenerated a test output signal with a negative current during the secondpulse of the test excitation signal; after the test pulse was reducedfrom about 400 mV to about 200 mV.

In 110 of FIG. 1, the biosensor compares the test output signal with oneor more underfill thresholds during the test period. The underfillthresholds may be predetermined threshold values stored in a memorydevice, obtained from a lookup table, or the like. The predeterminedthreshold values may have been developed from a statistical analysis oflaboratory work. Other predetermined threshold values may be used. Theunderfill thresholds may be measured or calculated threshold values inresponse to the test output signal. Other measured or calculatedthreshold values may be used.

The underfill thresholds may be selected to identify when a test outputsignal is stronger or weaker in response to an underfill condition. Theunderfill thresholds may be selected to identify weaker test outputsignals generated in response to a switch from a higher to loweramplitude in a test excitation signal. The underfill thresholds may beselected to identify negative test output signals generated in responseto a switch from a higher to lower amplitude in a test excitationsignal. The underfill thresholds may be selected to identify strongertest output signals generated in response to a switch from a lower tohigher amplitude in a test excitation signal. The underfill thresholdsmay be selected to identify when a change in a test output signal isresponsive to an underfill condition. Other underfill thresholds may beused.

The test output signal may indicate an underfill condition when the testoutput signal is equal to or less than a first underfill threshold. Thefirst underfill threshold may be predetermined threshold value stored ina memory device, obtained from a lookup table, or the like. The firstunderfill threshold may be a measured or calculated threshold value inresponse to the test output signal. The first underfill threshold may beless than about 50% or 75% of the expected or measured test outputsignal at the beginning of the first test pulse. The first underfillthreshold may be less than about 10% of the expected or measured testoutput signal at the beginning of the first test pulse. The firstunderfill threshold may be in the range of about 2% through about 8% ofthe expected or measured test output signal at the beginning of thefirst test pulse. The first underfill threshold may be in the range ofabout 5% of the expected or measured test output signal at the beginningof the first test pulse. The first underfill threshold may be aboutzero. For example, the first underfill threshold for the test outputsignals of FIGS. 9-12 may be in the range of about 100 nA through about200 nA. Other first underfill thresholds may be used.

The test output signal may indicate an underfill condition when a changein the test output signal is equal to or greater than a second underfillthreshold. The change may be a decrease in the test output signalgenerated in response to a switch from a higher to lower amplitude in atest excitation signal. The change may be an increase in the test outputsignal generated in response to a switch from a lower to higheramplitude in a test excitation signal. The second underfill thresholdmay be predetermined threshold value stored in a memory device, obtainedfrom a lookup table, or the like. The second underfill threshold may bea measured or calculated threshold value in response to the test outputsignal. The second underfill threshold may be greater than about 5% or10% of the expected or measured test output signal at the beginning ofthe first test pulse. The second underfill threshold may be in the rangeof about 5% through about 90% of the expected or measured test outputsignal at the beginning of the first test pulse. The second underfillthreshold may be in the range of about 25% through about 75% of theexpected or measured test output signal at the beginning of the firsttest pulse. The second underfill threshold may be about 50% of theexpected or measured test output signal at the beginning of the firsttest pulse. For example, the second underfill threshold for the testoutput signals of FIGS. 9-12 may be in the range of about 500 nA throughabout 2000 nA. Other second underfill thresholds may be used.

The test output signal may indicate an underfill condition when the testoutput signal is equal to or greater than a third underfill threshold.The third underfill threshold may be predetermined threshold valuestored in a memory device, obtained from a lookup table, or the like.The third underfill threshold may be a measured or calculated thresholdvalue in response to the test output signal. The third underfillthreshold may be greater than about 150% or 200% of the expected ormeasured test output signal at the beginning of the first test pulse.The third underfill threshold may be greater than about 110% of theexpected or measured test output signal at the beginning of the firsttest pulse. The third underfill threshold may be in the range of about102% through about 108% of the expected or measured test output signalat the beginning of the first test pulse. The third underfill thresholdmay about 105% of the expected or measured test output signal at thebeginning of the first test pulse. Other third underfill thresholds maybe used.

FIG. 13 is a graph illustrating the percent bias of analyte analyses inrelation to the volume of a sample. The analyte analyses determined theconcentration of glucose in samples of whole blood. The percent of bias(%-bias) is an error measurement of the relative difference between theglucose concentration determined by each analysis and the glucoseconcentration of the sample when sufficiently filled. The sample volumeswere in a range of about 1.2 μL through about 2.0 μL. A sufficientlyfilled sample volume was about 2.0 μL.

The test output signals from the analyte analysis were screened by twofilters (Filter 1 and Filter 2) to identify samples with underfillconditions. A Filter 1 (F1) error indicates the sample has an underfillcondition when the test output signal is equal to or less than a firstunderfill threshold. A Filter 2 (F2) error indicates the sample has anunderfill condition when a decrease in the test output signal at orafter the transition from a higher to lower test pulse is equal to orgreater than a second underfill threshold. Other filters may be used.

There were three types of test output signals from the analyses in FIG.13: (1) test output signals indicating no F1 errors; (2) test outputsignals indicating a F1 error; and (3) test output signals indicating noF1 error, but indicating a F2 error. Of the test output signalsindicating no F1 errors, only four analyses had a %-bias greater thanabout ±15%. Three of the analyses with a %-bias greater than about ±15%and not detected as F1 errors were detected as F2 errors.

FIG. 14 is a graph illustrating the percent population of differenttypes of test output signals in relation to the volume of a sample forthe analyte analyses of FIG. 13. The percent population (%-population)is the proportion of the analyte analyses having a particular type oftest output signal at a sample volume. The analyte analyses with testoutput signals having F1 or F2 errors were essentially exclusive fromthe analyte analyses with a %-bias less than ±15%. Essentially, thoseanalyte analyses with test output signals not screened out by F1 or F2errors had a %-bias less than ±15%. The detection rate was greater thanabout 98% for underfill conditions of analyses with a %-bias greaterthan about ±15%. The detection rate was greater than about 90% forunderfill conditions of analyses with a %-bias greater than about ±10%.The detection rates may be further refined with different thresholdvalues. Factors other than underfill may contribute to %-bias greaterthan ±15%.

In 112 of FIG. 1, the biosensor generates an error signal or otherindication in response to an underfill condition when the test outputsignal indicates the sample size is not large enough. The error signalmay be shown on a display device and/or retained in a memory device. Thebiosensor may provide the error signal during or after the analysis ofone or more analytes in the sample is performed. The biosensor mayprovide the error signal immediately after detection and/or prior to theanalysis of the analyte. The error signal may request the addition ofbiological fluid to the sample prior to proceeding with the analysis ofthe analyte. The error signal may stop the analysis of the analyte. Stopincludes not starting or suspending the analysis.

FIG. 15 depicts a schematic representation of a biosensor 1500 with anunderfill detection system. Biosensor 1500 determines an analyteconcentration in a sample of a biological fluid. The underfill detectionsystem indicates when a sample of the biological fluid is not largeenough to provide an accurate and/or precise analysis of one or moreanalytes as previously discussed. Biosensor 1500 includes a measuringdevice 1502 and a sensor strip 1504, which may be implemented as abench-top device, a portable or hand-held device, or the like. Themeasuring device 1502 and sensor strip 1504 may be adapted to implementan electrochemical sensor system, an optical sensor system, acombination thereof, or the like. The underfill detection system mayimprove the accuracy and/or precision of the biosensor 1500 indetermining when underfill conditions occur. Biosensor 1500 may beutilized to determine one or more analyte concentrations, such asglucose, uric acid, lactate, cholesterol, bilirubin, or the like, in abiological fluid, such as whole blood, urine, saliva, or the like. Whilea particular configuration is shown, biosensor 1500 may have otherconfigurations, including those with additional components.

The sensor strip 1504 has a base 1506 that forms a reservoir 1508 and achannel 1510 with an opening 1512. The reservoir 1508 and channel 1510may be covered by a lid with a vent. The reservoir 1508 defines apartially-enclosed volume (the cap-gap). The reservoir 1508 may containa composition that assists in retaining a liquid sample such aswater-swellable polymers or porous polymer matrices. Reagents may bedeposited in the reservoir 1508 and/or channel 1510. The reagents mayinclude one or more enzymes, mediators, binders, and other active ornon-reactive species. The reagents may include a chemical indicator foran optical system. The sensor strip 1504 also may have a sampleinterface 1514 disposed adjacent to the reservoir 1508. The sampleinterface 1514 may partially or completely surround the reservoir 1508.The sensor strip 1504 may have other configurations.

The sample interface 1514 has conductors connected to a workingelectrode and a counter electrode. The electrodes may be substantiallyin the same plane. The electrodes may be separated by greater than 200or 250 μm and may be separated from the lid by at least 100 μm. Theelectrodes may be disposed on a surface of the base 1506 that forms thereservoir 1508. The electrodes may extend or project into the cap-gapformed by the reservoir 1508. A dielectric layer may partially cover theconductors and/or the electrodes. The counter electrode may have asub-element or trigger electrode. The sub-element may be locatedupstream from the working electrode. The trigger electrode may be athird electrode. The sample interface 1514 may have other electrodes andconductors. The sample interface 1514 may have one or more opticalportals or apertures for viewing the sample. The sample interface 1514may have other components and configurations.

The measuring device 1502 includes electrical circuitry 1516 connectedto a sensor interface 1518 and a display 1520. The electrical circuitry1516 includes a processor 1522 connected to a signal generator 1524, anda storage medium 1528. The measuring device may have other componentsand configurations.

The signal generator 1524 provides electrical input signals to thesensor interface 1518 in response to the processor 1522. The electricalinput signals may include the polling and test excitation signals usedin the underfill detection system. The electrical input signals mayinclude electrical signals used to operate or control a detector andlight source in the sensor interface 1518 for an optical sensor system.The electrical input signals may include an assay excitation signal usedin an electrochemical sensor system. The polling and test excitationsignals for the underfill detection system may be part of orincorporated with the assay excitation signal for an electrochemicalsensor system. The electrical input signals may be transmitted by thesensor interface 1518 to the sample interface 1514. The electrical inputsignals may be a potential or current and may be constant, variable, ora combination thereof, such as when an AC signal is applied with a DCsignal offset. The electrical input signals may be applied as a singlepulse or in multiple pulses, sequences, or cycles. The signal generator1524 also may record signals received from the sensor interface 1518 asa generator-recorder.

The storage medium 1528 may be a magnetic, optical, or semiconductormemory, another computer readable storage device, or the like. Thestorage medium 1528 may be a fixed memory device or a removable memorydevice such as a memory card.

The processor 1522 implements the underfill detection, analyte analysis,and data treatment using computer readable software code and data storedin the storage medium 1528. The processor 1522 may start the underfilldetection and analyte analysis in response to the presence of sensorstrip 1504 at the sensor interface 1518, the application of a sample tothe sensor strip 1504, user input, or the like. The processor 1522directs the signal generator 1524 to provide the electrical inputsignals to the sensor interface 1518.

The processor 1522 receives and measures output signals from the sensorinterface 1518. The output signals may be electrical signals, such ascurrent or potential, or light. The output signals may include thepolling and test output signals used in the underfill detection system.The output signals may include an assay output signal generated inresponse to the redox reaction of the analyte in the sample. The outputsignal may be generated using an optical system, an electrochemicalsystem, or the like. The test output signal for the underfill detectionsystem may be part of or incorporated with the assay output signal foran electrochemical sensor system. The processor 1522 may compare thepolling output signals to one or more polling thresholds as previouslydiscussed. The processor 1522 may compare the test output signals to oneor more underfill thresholds as previously discussed.

The processor 1522 provides an error signal or other indication of anunderfill condition when the test output signal indicates the samplesize is not large enough. The processor 1522 may display the errorsignal on the display 1520 and may store the error signal and relateddata in the storage medium 1528. The processor 1522 may provide theerror signal at any time during or after the analyte analysis. Theprocessor 1522 may provide the error signal when an underfill conditionis detected and may prompt a user to add more of the biological fluid tothe sensor strip 1204. The processor 1522 may not proceed with theanalyte analysis when an underfill condition is detected.

The processor 1522 determines analyte concentrations from the assayoutput signals. The results of the analyte analysis are output to thedisplay 1520 and may be stored in the storage medium 1528. Instructionsregarding implementation of the analyte analysis may be provided by thecomputer readable software code stored in the storage medium 1528. Thecode may be object code or any other code describing or controlling thedescribed functionality. The data from the analyte analysis may besubjected to one or more data treatments, including the determination ofdecay rates, K constants, slopes, intercepts, and/or sample temperaturein the processor 1522.

The sensor interface 1518 has contacts that connect or electricallycommunicate with the conductors in the sample interface 1514 of thesensor strip 1504. The sensor interface 1518 transmits the electricalinput signals from the signal generator 1524 through the contacts to theconnectors in the sample interface 1514. The sensor interface 1518 alsotransmits the output signals from the sample interface 1514 to theprocessor 1522 and/or signal generator 1524. The sensor interface 1508also may include a detector, a light source, and other components usedin an optical sensor system.

The display 1520 may be analog or digital. The display may be an LCDdisplay adapted to displaying a numerical reading. Other displays may beused.

In use, a liquid sample of a biological fluid is transferred into thecap-gap formed by the reservoir 1508 by introducing the liquid to theopening 1512. The liquid sample flows through channel 1510 intoreservoir 1508, filling the cap-gap while expelling the previouslycontained air. The liquid sample chemically reacts with the reagentsdeposited in the channel 1510 and/or reservoir 1508.

The processor 1522 detects when the sample of the biological fluid isavailable for analysis. The sensor strip 1502 is disposed adjacent tothe measuring device 1502. Adjacent includes positions where the sampleinterface 1514 is in electrical and/or optical communication with thesensor interface 1508. Electrical communication includes the transfer ofinput and/or output signals between contacts in the sensor interface1518 and conductors in the sample interface 1514. Optical communicationincludes the transfer of light between an optical portal in the sampleinterface 1502 and a detector in the sensor interface 1508. Opticalcommunication also includes the transfer of light between an opticalportal in the sample interface 1502 and a light source in the sensorinterface 1508.

The processor 1522 may direct the signal generator 1524 to provide apolling excitation signal to sensor interface 1518, which applies thepolling excitation signal to the sample through the electrodes in thesample interface 1514. The sample generates the polling output signal inresponse to the polling excitation signal. The sample interface 1514provides the polling output signal to the sensor interface 1518. Theprocessor 1522 receives the polling output signal from the sensorinterface 1518. The processor 1522 may show the polling output signal onthe display 1520 and/or may store the polling output signal in thestorage medium 1528.

The processor 1522 may direct the signal generator 1524 to provide thetest excitation signal to the sensor interface 1518 when the pollingoutput signal is equal to or greater than a polling threshold. Theprocessor 1522 may have comparator circuitry to provide the testexcitation signal to the sensor interface 1518 when the polling outputsignal is equal to or greater than a polling threshold. In thecomparator circuitry, the polling output signal is directed into theinput of an electrical (analog) comparator or the like. The comparatorcompares the polling output signal with a polling threshold value. Whenthe polling output signal is equal to or greater than the pollingthreshold value, the output of the comparator triggers the launch of thetest excitation signal. When switching from the polling excitationsignal to the test excitation signal, the processor 1522 may change theamplitude of the test pulses to a different amplitude than the amplitudeof the polling pulses as previously discussed. The amplitude of the testpulses may be larger and/or smaller than the amplitude of the pollingpulses. The amplitude of one test pulse may be larger or smaller thanthe amplitude of another test pulse. The processor 1522 may change theamplitude of the test pulses at or near the start of the test excitationsignal and/or during a transition from one pulse to another. Theprocessor 1522 may change the amplitude of the test pulses multipletimes.

The sensor interface 1518 applies the test excitation signal to thesample through the sample interface 1514 during a test period. Thesample generates the test output signal in response to the testexcitation signal. The sample interface 1514 provides the test outputsignal to the sensor interface 1518.

The processor 1522 receives the test output signal from the sensorinterface 1518. The processor 1522 measures the test output signalgenerated by the sample. The processor 1522 may show the test outputsignal on the display 1520 and/or may store test output signal in thestorage medium 1528.

The processor 1522 compares the test output signal with one or moreunderfill thresholds during the test period as previously discussed. Thetest output signal may indicate an underfill condition when the testoutput signal is equal to or less than a first underfill threshold. Thetest output signal may indicate an underfill condition when a change inthe test output signal is equal to or greater than a second underfillthreshold. The test output signal may indicate an underfill conditionwhen the test output signal is equal to or greater than a thirdunderfill threshold.

The processor 1522 provides an error signal of an underfill conditionwhen the test output signal indicates the sample size is not largeenough. The error signal may be shown on the display 1520 and/orretained in the storage medium 1528. The processor 1522 may provide theerror signal immediately or another time, such as after the analyteanalysis. The processor 1522 may prompt a user to add more biologicalfluid to the sample prior to proceeding with the analysis of theanalyte.

The processor 1522 directs the signal generator 1524 to provide theother electrical input signals to the sensor interface 1518. In anoptical system, the sensor interface 1518 provides the electrical inputsignals to operate the detector and light source. The sensor interface1518 receives the assay output signal from the detector. In anelectrochemical system, the sensor interface 1518 applies the assayexcitation signal to the sample through the sample interface 1514. Thetest excitation signal for the underfill detection system may be part ofor incorporated with the assay excitation signal. The sample generatesthe assay output signal from the redox reaction of the analyte inresponse to the assay excitation signal. The sample interface 1514provides the assay output signal to the sensor interface 1518.

The processor 1522 receives the assay output signal from the sensorinterface 1518. The processor 1522 determines the analyte concentrationof the sample in response to the assay output signal. The processor 1522may show the assay output signal on the display 1520 and/or may storethe assay output signal in the storage medium 1528.

Without limiting the scope, application, or implementation, the methodsand systems previously described may be implemented using an algorithm,such as the following:

Step 1: Turn on biosensor power Step 2: Perform biosensor self-test Step3: Setup to poll for application of sample to sensor Step 4: Setup fortesting the sensor current Step 5: Test if the sensor current exceedsthe polling threshold Step 6: Delay and test sensor current again Step7: Upon detection of Sample Application    start counting time    launchpulse sequence Step 8: Pulse 1 - Measure sensor currents i_(1,1) andi_(1,8) Step 9: Pulse 2 - Measure sensor currents i_(2,1) and i_(2,8)Step 10: Delay 2 - Step 11: Pulse 3 - Measure sensor currents: i_(3,1)and i_(3,8) Step 12: Delay 3 - Step 13: Pulse 4 - Measure sensorcurrents: i_(4,1,) i_(4.4,) and i_(4,8) Step 14: Delay 4 - Step 15:Pulse 5 - Measure sensor currents: i_(5,1,) i_(5,4,) and i_(5,8) Step16: Look up slope and intercept for lot calibration number    S = Slopevalue for current lot calibration number    Int = Intercept value forcurrent lot calibration    number Step 17: Adjust slope and interceptfor temperature effect Step 18: Calculate glucose concentration at 25°C. Step 19: Convert to target reference (plasma vs. WB reference) Step20: Check underfill If (i_(2,8) < Underfill_(Min) ) or ( (i_(1,8) −i_(2,1)) > Underfill_(Delta) ) then       BEGIN If (ErrorCode is notset) then set ErrorCode to “Underfill”       END Step 21: Convert tocorrect units of measure setting Step 22: Display result

One example of the constants that may be used in the algorithm is givenin Table I below. Other constants may be used.

TABLE I Constant Description Value Units Underfill_(Min) currentthreshold for underfill 150 nA check, 1^(st) criteria Underfill_(Delta)current delta threshold for 700 nA underfill, 2^(nd) criteria

To provide a clear and more consistent understanding of thespecification and claims of this application, the following definitionsare provided.

“Analyte” is defined as one or more substances present in a sample. Ananalysis determines the presence and/or concentration of the analytepresent in the sample.

“Sample” is defined as a composition that may contain an unknown amountof the analyte. Typically, a sample for electrochemical analysis is inliquid form, and preferably the sample is an aqueous mixture. A samplemay be a biological sample, such as blood, urine, or saliva. A samplealso may be a derivative of a biological sample, such as an extract, adilution, a filtrate, or a reconstituted precipitate.

“Conductor” is defined as an electrically conductive substance thatremains stationary during an electrochemical analysis.

“Accuracy” is defined as how close the amount of analyte measured by asensor system corresponds to the true amount of analyte in the sample.Accuracy may be expressed in terms of the bias of the sensor system'sanalyte reading in comparison to a reference analyte reading. Largerbias values reflect less accuracy.

“Precision” is defined as how close multiple analyte measurements arefor the same sample. Precision may be expressed in terms of the spreador variance among multiple measurements.

“Redox reaction” is defined as a chemical reaction between two speciesinvolving the transfer of at least one electron from a first species toa second species. Thus, a redox reaction includes an oxidation and areduction. The oxidation half-cell of the reaction involves the loss ofat least one electron by the first species, while the reductionhalf-cell involves the addition of at least one electron to the secondspecies. The ionic charge of a species that is oxidized is made morepositive by an amount equal to the number of electrons removed.Likewise, the ionic charge of a species that is reduced is made lesspositive by an amount equal to the number of electrons gained.

“Mediator” is defined as a substance that may be oxidized or reduced andthat may transfer one or more electrons. A mediator is a reagent in anelectrochemical analysis and is not the analyte of interest, butprovides for the indirect measurement of the analyte. In a simplesystem, the mediator undergoes a redox reaction in response to theoxidation or reduction of the analyte. The oxidized or reduced mediatorthen undergoes the opposite reaction at the working electrode of thesensor strip and is regenerated to its original oxidation number.

“Binder” is defined as a material that provides physical support andcontainment to the reagents while having chemical compatibility with thereagents.

“Underfill condition” is defined as a sample of biological fluid in abiosensor having a size or volume that is not large enough for thebiosensor to accurately and/or precisely analyze the concentration ofone or more analytes in the biological fluid.

“Handheld device” is defined as a device that may be held in a humanhand and is portable. An example of a handheld device is the measuringdevice accompanying Ascensia® Elite Blood Glucose Monitoring System,available from Bayer HealthCare, LLC, Elkhart, Ind.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that other embodimentsand implementations are possible within the scope of the invention.

What is claimed is:
 1. A biosensor for determining an analyte concentration in a biological fluid, comprising: a sensor strip having a sample interface on a base, the base forming a reservoir and a channel with an opening, the base further having a surface on which at least one working electrode and at least one counter electrode are disposed, the sample interface having conductors connected to the working electrode and the counter electrode, a biological fluid sample received in the opening flowing through the channel to fill at least in part the reservoir; a measuring device in electrical or optical communication with the reservoir, the measurement device having electrical circuitry including a processor and a storage medium, the electrical circuitry being communicatively coupled to a signal generator and a sensor interface, the processor having instructions and data stored in the storage medium and being configured to: in response to receiving the biological fluid sample in the reservoir, instruct the signal generator to apply a test excitation electrical signal to the sensor interface, the test excitation electrical signal being transmitted by the sensor interface to the sample interface for applying the test excitation electrical signal to the biological fluid sample, switch the test excitation electrical signal to at least one different amplitude during at least one of a start of the test excitation electrical signal, a test pulse, or a transition from one test pulse to another test pulse, in response to the test excitation electrical signal, generate from the biological fluid sample a first test output electrical signal during a first time interval and a second test output electrical signal during a second time interval, the first time interval being different than the second time interval, subtract a value of the first test output electrical signal from a value of the second test output electrical signal to determine a value change between the first test output electrical signal and the second test output electrical signal, compare the value change to a value of at least one underfill threshold, and determine if the sensor strip is underfilled based on the comparison between the value change and the at least one underfill threshold.
 2. The biosensor of claim 1, wherein the processor applies a polling excitation electrical signal to the biological fluid sample.
 3. The biosensor of claim 2, wherein the processor switches from the polling excitation electrical signal to the test excitation electrical signal when the polling excitation electrical signal is equal to or greater than a polling threshold.
 4. The biosensor of claim 2, wherein the processor applies the polling excitation electrical signal during a polling period of less than 180 seconds, and wherein the processor applies the test excitation electrical signal during a test period of less than 180 seconds.
 5. The biosensor of claim 1, wherein the test excitation electrical signal has a test pulse width less than about 5 sec and a test pulse interval less than about 15 sec.
 6. The biosensor of claim 1, wherein the at least one different amplitude is lower than an original amplitude, and wherein the original and different amplitudes are selected from an output signal plateau in an electrochemical sensor system.
 7. The biosensor of claim 6, wherein the output electrical signal plateau includes excitation amplitudes that generate output electrical signals within ±5% of an average output electrical signal.
 8. The biosensor of claim 1, wherein the processor switches the test excitation electrical signal to a first different amplitude during a test pulse, and wherein the processor switches the test excitation electrical signal to a second different amplitude during a transition from one test pulse to another test pulse.
 9. The biosensor of claim 1, wherein the processor reduces the amplitude of at least one test pulse in the test excitation electrical signal below the amplitude of a polling pulse in the polling excitation electrical signal.
 10. The biosensor of claim 1, further comprising a display connected to the processor, wherein the processor shows an error signal on the display in response to an underfill condition.
 11. The biosensor of claim 10, wherein the error signal requests the addition of biological fluid to the sample in response to the error signal.
 12. The biosensor of claim 10, wherein the processor stops an analysis of an analyte in a sample in response to the error signal.
 13. A method for detecting an underfill condition in a biosensor, comprising: providing a biosensor system in form of an analytical instrument including a sensor strip having a sample interface on a base, the base forming a reservoir and a channel with an opening, the base further having a surface on which at least one working electrode and at least one counter electrode are disposed, the sample interface having conductors connected to the working electrode and the counter electrode, a biological fluid sample received in the opening flowing through the channel to fill at least in part the reservoir, and a measuring device in electrical or optical communication with the reservoir, the measurement device having electrical circuitry including a processor and a storage medium, the electrical circuitry being communicatively coupled to a signal generator and a sensor interface, the processor having instructions and data stored in the storage medium; in response to receiving the biological fluid sample in the reservoir, instructing the signal generator, via the processor, to apply a test excitation electrical signal to the sensor interface; transmitting, via the processor, the test excitation electrical signal by the sensor interface to the sample interface, the test excitation electrical signal including at least first and second test pulses separated by a test relaxation; switching the test excitation electrical signal to at least one different amplitude during at least one of a start of the text excitation electrical signal, a test pulse, or a transition from one test pulse to another test pulse, in response to the test excitation electrical signal, generating, via the processor, during a first time interval a first output electrical signal from the first test pulse and during a second time interval a second test output electrical signal from the second test pulse, the first time interval being different than the second time interval; measuring, via the processor, the first test output signal from the first test pulse; measuring, via the processor, the second test output signal from the second test pulse; subtracting, via the processor, a value of the first test output electrical signal from a value of the second test output electrical signal to determine a value change between the first test output electrical signal and the second test output electrical signal; comparing, via the processor, the value change to a value of at least one underfill threshold; and detecting, via the processor, if the sensor strip is underfilled based on the comparing between the value change and the at least one underfill threshold. 